UNLABELLED: Standardised triage systems have been implemented in emergency departments (EDs) to improve the efficacy of assessment strategies as performed by registered nurses (RNs). However, the exact effect the standardised triage systems have on the decision-making process remains unclear. AIM: To evaluate decision making in the triage setting before and after implementation of the Medical Emergency Triage and Treatment System Adult in one hospital's ED. METHODS: A descriptive intervention design with a quantitative approach. A total of 655 patients before and 413 patients after the intervention were included. A questionnaire was used to evaluate how the RNs assessed the patients before intervention while the emergency patient records were used for data collection after intervention. RESULTS: Before the intervention, a majority of the assessments were founded on signs and symptoms and medical diagnoses, whereas vital parameters were rarely used. After the intervention, nearly two thirds of the patients were assessed according to a triage system with vital parameters and standardised algorithm for symptoms and signs included in the assessment procedure. CONCLUSION: Implementing a standardised triage system, including vital parameters and standardised algorithms for signs and symptoms, increased the use of vital parameters and signs and symptoms for decision making and acuity assignment.